This application is a continuation of Internaional Application No. PCT/JP00/04283, filed Jun. 28, 2000, the disclosure of which is incorporated herein by reference.
The present invention relates to a non-aqueous electrolyte secondary battery. More particularly, the present invention relates to a non-aqueous electrolyte secondary battery having a highly reliable negative electrode with a high electric capacity and with the growth of dendrite suppressed.
High voltage and high energy density can be expected from non-aqueous electrolyte secondary batteries using lithium or a lithium compound in the negative electrode. Positive electrode active materials for non-aqueous electrolyte secondary batteries that have been known include oxides and chalcogens of transition metals such as LiMn2O4, LiCoO2, LiNiO2, V2O5, Cr2O5, MnO2, TiS2 and MoS2. Those compounds have a layered or tunneled crystal structure that allows intercalation and deintercalation of lithium ions. As to the negative electrode active material, on the other hand, there are many previous studies on metallic lithium. However, the use of metallic lithium causes dendrite to deposit on the surface of lithium during charging, which reduces charge/discharge efficiency. In addition, internal short-circuit is caused if dendrites come in contact with the positive electrode.
As a solution to those problems, the use of a lithium alloy such as lithium-aluminum alloy, which not only suppresses the growth of dendrite but also can absorb lithium therein and desorb it therefrom in the negative electrode, has been studied. However, in case the lithium alloy is used, the repeating of deep charging and discharging causes pulverization of the electrode, presenting a problem concerning the cycle life characteristics.
In recent years, a highly safe carbon material capable of reversibly absorbing and desorbing lithium and excellent in cycle life characteristics has been used in the negative electrode, although smaller in capacity than metallic lithium or lithium alloy. In an attempt to further increase the capacity of the non-aqueous electrolyte secondary battery, studies have been made about application of oxides to the negative electrode. For example, it is suggested in Japanese Laid-Open Patent Publications Hei 7-122274 and Hei 7-235293 that crystalline SnO and SnO2 may serve as negative electrode materials having high capacities than the conventional oxide WO2. It is also proposed in Japanese Laid-Open Patent Publication Hei 7-288123 that non-crystalline oxide such as SnSiO3 or SnSi1xe2x88x92xPxO3 is used in the negative electrode in order to improve the cycle life characteristics of the battery. However, no sufficient characteristics have been obtained yet.
In view of the circumstance described above, it is the object of the present invention to provide a non-aqueous electrolyte secondary battery having a high capacity and excellent charge/discharge cycle life characteristics in which no dendrite grows because the negative electrode absorbs lithium upon charging.
The present invention relates to a non-aqueous electrolyte secondary battery comprising a chargeable and dischargeable positive electrode, a non-aqueous electrolyte and a chargeable and dischargeable negative electrode, wherein the negative electrode has a solid solution, as an active material, the solid solution being represented by the formula (1):
LixM1yM2zxe2x80x83xe2x80x83(1)
wherein M1 represents at least one element selected from the group consisting of Ti, Zr, Mn, Co, Ni, Cu and Fe, and M2 represents at least one element selected from the group consisting of Si and Sn, and wherein 0xe2x89xa6x less than 10, 0.1xe2x89xa6yxe2x89xa610 and z=1.
The average particle size of the solid solution represented by the formula (1) is preferably 0.5 to 2.3 xcexcm. And the average crystal grain size of the solid solution represented by the formula (1) is preferably 0.05 to 0.13 xcexcm.
The above-mentioned negative electrode contains preferably 5 to 50 parts by weight of carbon material per 100 parts by weight of the solid solution represented by the formula (1).